Spectrometer is an non-destructive detection equipment for characterization of substance composition and properties. In a spectral measurement, lights would be irradiated onto the target substance, and internal structure of the substance would result in reflection, adsorption or permission of the lights of different wavelengths. Upon reception of the lights reflected by or passed through the substance by the spectrometer, a spectrum of the substance would be obtained. As every substance has a distinct spectral feature, composition and properties of different substances may be characterized.
To reduce light loss, waveguide device is usually adopted to guide travel of lights in the internal passages of the spectrometer to allow the image sensor to detect the spectrum generated by the diffraction grating. Conventionally, the image sensor is disposed adjacent to the waveguide device to minimize light loss. However, inventors of the present invention observed that such disposition of the image sensor is associated with tailing effect that could reduce spectral resolution and affect accuracy of the resulting spectral data, such as peak intensities and wavelengths.
The tailing effect is caused by astigmatism of the grating, and is discussed in Diffraction Grating Handbook, 6th edition authored by Christopher Palmer of Newport Corporation (specifically, in first paragraph in page 90, FIGS. 7-3 in page 94, third paragraph in page 111, and FIGS. 8-1b in page 112). When a cylindrical grating is adopted as the concave grating, light beams could be focused only along the radial plane (that is, the tangential plane, or essentially the horizontal plane) but not along the vertical plane (that is, the sagittal plane). As such, when lights incident the grating at an oblique angle, the tangential focus projected from the waveguide device would exhibit a moon-like shape. Such phenomenon has been referred to as the tailing effect. Unlike a spot focus, the moon-shaped focus would reduce spectral resolution. Moreover, being detectable by pixels of the corresponding wavelength surrounding those of the image sensor, the moon-shaped focus would also result in inaccuracy of the acquired spectral data, such as peak intensities and wavelengths.